(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery.
(b) Description of the Related Art
Although research to develop a negative active material having a high capacity based on metallic materials such as Si, Sn, and Al has actively been undertaken, such research has not yet succeeded in applying said metals to a negative active material. This is mainly due to problems in that the cycle characteristics are deteriorated by a series of processes of intercalating and deintercalating lithium ions with metals such as Si, Sn, and Al, and the consequential expansion and contraction of the volume thereof, which pulverizes the metal to a fine powder. In order to attempt to solve these problems, an amorphous alloy oxide has been suggested by Y. Idota, et al: Science, 276, 1395(1997). In addition, it is reported that a negative active material comprising an amorphous structured alloy improves a battery's cycle characteristics in 43rd Preview of Battery Discussion (The Electrochemical Society of Japan, The Committee of Battery Technology, Oct. 12, 2002, p. 308-309).
Although Si is expected to provide a higher capacity, Si is generally known to be too hard to be transferred to an amorphous phase either by itself or in an Si-alloy form. However, recently, it has been reported that Si material can be transferred into amorphous phase via a mechanical alloying process.
As mentioned in 43rd Preview of Battery Discussion (The Electrochemical Society of Japan, The Committee of Battery Technology, Oct. 12, 2002, p. 308-309), amorphous alloy material has a good early stage capacity retention rate relative to that of crystalline alloy material, but that capacity tends to remarkably decrease after repeated charge-discharge cycles. For amorphous material, as it does not have the same structure as a crystal material, the expansion rate upon charging is relatively low and the characteristics deteriorate less upon repeated charge and discharge compared to those for crystal material. In addition, the amorphous material can improve the early stage cycle characteristics better than crystal material because the lithium ion is better diffused. Further, although the active material is not fully charged in the very early stage, the utilization of an active material is slowly increased upon repeating cycles and, as a result, the deterioration of the cycle characteristics due to the pulverization of the material to a fine powder is alleviated. However, upon repeating the cycles, it is anticipated that the cycle characteristics will deteriorate due to the pulverization of the material to a fine powder and the exhaustion of the active material.
For a mechanical alloying process, a pulverizing step into fine powder and a compressing step are repeated to slowly reduce the crystal degree to provide an amorphous or pulverized material. However, such a process may cause problems in that the interface is broken between the tiny alloy structures identified via a X-ray diffraction analysis, and the structure is easily broken upon intercalating lithium ions and pulverized. Thereby the cycle characteristics deteriorate.